One type of continuous tubular emulsion polymerization reactor comprises a first section with a closed circulation loop, a driving means, e.g. a circulation pump, for circulating a reactor charge within the circulation loop, and a discharge connected to a secondary line section. Water phase, or another suitable carrier, monomers, and optionally stabilizers are continuously fed to the loop and circulated. Polymer emulsion is continuously drawn off at the same rate at which the water phase, monomers and initiators (if used) are fed into the reactor.
Such types of reactors are discussed in WO 2007/031478, incorporated herein by reference. This publication describes that such reactors are particularly suitable for the production of polymers derived from vinyl and/or acrylic monomers, used for instance in paints or adhesives.
However, a problem encountered in polymerization processes employing a tubular reactor, including the one described above, is the formation of deposits or “fouling” from the reaction products on the internal wall of the reactor. These deposits lead to a need for an increased delivery pressure from the circulation pump and impair heat transfer from the reaction medium to, e.g., a coolant in a jacket surrounding the reactor tube, thus leading to higher (and often deleterious) reactor temperatures or necessitating either an increased coolant circulation rate, a lower coolant temperature, or a reduced rate of production so as to match the heat removal.
A further general disadvantage of a fouled reactor is the reduction of volume, increasing the shear on the emulsion. This reduction in volume shifts the process conditions which may have been optimized on a clean reactor. In any case, product properties will drift, nullifying the advantages of consistency of production expected from continuous reactors.
One way of cleaning the inside of tubes or pipework is the use of a cleaning member or pig which is forced through the tube. German patent application 3233557, for instance, describes various ways of using a pig for cleaning the internal wall of a tubular reactor. In one embodiment (illustrated in FIG. 1 of the application), two ball valves are provided in the reactor tube for receiving and launching one or more pigs. The reaction product is used to drive the pig and leaves the reactor after the pig has passed the outlet. Thus, the reaction product is not and cannot be re-circulated, rendering this configuration unsuitable for continuous polymerization. In a further embodiment (FIG. 2), scraper pigs may be passed through the pump. This method imposes severe limitations on the shape of the pig and the type of pump used. For instance, the use of a positive displacement type pump is not possible.
It is noted that U.S. Pat. No. 3,425,083 discloses an endless pipe which has a bent form allowing a cleaning member to circulate constantly through the pipe and past its inlet and outlet for a liquid medium. The outlet has a smaller capacity than the inlet, so that part of the liquid medium is returned from the outlet through a return portion of the pipe to and past the inlet for recycling. No means are provided for removing the cleaning member or for interrupting its circulation. Further, the pipe is intended for heating liquids like milk and not for cooling reacting emulsions. In fact, reactors are not mentioned at all.
U.S. Pat. No. 3,682,186 discloses an apparatus for by-passing scrapers or product displacers around a pipeline booster or compression station. The main line is provided with two check valves for receiving and launching a pig. Loop reactors are not mentioned.
Thus a continuous tubular emulsion polymerization reactor utilizing a pig system that allows for automated reactor cleaning without shutting down the polymerization process would be beneficial. In addition, for a two stage reactor with a circulation loop stage and a secondary line stage, it would be useful if both stages could be pigged by a single pig without interruption or manual intervention to retrieve the pig for the secondary stage cleaning. Such an automated system would be more efficient and eliminate the safety concerns introduced by a manual intervention to retrieve the pig.